X-ray imaging systems are widely used in the medical field. The X-ray imaging system takes images of an internal structure a patient (subject) using X-ray. Absorption and scattering rates of X-rays vary depending on the organs and the thickness of the body of a patient. X-ray attenuates while it passes through the patient, reflecting the internal structure of the patient. In a conventional X-ray imaging system, a patient is placed between an X-ray source and a film to which a photosensitive agent is applied, and X-ray which passed through the patient is recorded on the film. Thus, an X-ray image of an internal structure of the patient is taken.
Recently, CR (computed radiography) devices have come into widespread use. In place of an X-ray film, the CR device uses a so-called imaging plate coated with photostimulable phosphor to temporarily record a latent image, and reads the recorded image with laser and digitizes the image. Moreover, digital radiography (DR or DX) devices are becoming available. The DR device converts X-ray, transmitted through the patient, into electric signals in real time and digitizes the image with the use of a flat panel detector (FPD).
The FPD is formed by layering semiconductor material such as amorphous selenium or the like on a large glass substrate, and converts incident X-ray into signal charge. The FPD has sensing elements corresponding to pixels. The signal charge from each pixel is stored in the corresponding sensing element. The signal charge stored in the FPD is output as image signals. The image signals are subjected to various image processing to generate a digital image.
Since the size of the FPD is extremely large compared to imaging sensors such as CCD and CMOS used for digital cameras, it is difficult to manufacture all the sensing elements with perfectly uniform properties. It is known that a degree of deterioration in each sensing element differs depending on the X-ray exposure record and the usage environment. Noise caused by defects of the sensing elements and time-varying deterioration of the sensing elements depending on the usage history of the FPD is referred to as offset noise. Since the offset noise is caused by deterioration of the FPD with time, once the offset noise occurs, it appears in every taken image from then on regardless of the time interval (lapse) between X-ray exposures. Therefore, to take an image using the FPD, an image (offset image) representing noise caused by defects and the time-varying deterioration of the sensing elements is obtained by calibration prior to the X-ray exposure (image-taking), and this image is subtracted from the taken image. Thus, the offset noise is removed from the taken image.
To take a highly reliable image with the FPD, it is preferable to calibrate the FPD just before the X-ray exposure and use the most accurate offset image possible. For example, an X-ray imaging system which automatically performs calibration based on time elapsed from the previous calibration and the number of X-ray exposures performed so as to acquire the most accurate offset image possible is known (see U.S. Pat. No. 6,476,394 corresponding to Japanese Patent Laid-Open Publication No. 2001-149355).
Signal charge of the previous image may be trapped by impurity level of the sensing elements and remain in the FPD after the X-ray exposure. Such residual signal charge in the FPD is superimposed onto the subsequent image as a residual image. The residual image attenuates after a long time. However, when the images are taken successively in short time intervals, the residual image is superimposed onto the subsequent image before it attenuates fully.
An X-ray imaging system using an imaging plate as an imaging device disclosed in U.S. Pat. No. 7,199,389 (corresponding to Japanese Patent Laid-Open Publication No. 2005-283798), for example, also causes residual images. In this X-ray imaging system, the residual image of the previous image is removed from the present image in consideration of attenuation based on time elapsed from the previous X-ray exposure.
An X-ray imaging apparatus of Japanese Patent Laid-Open Publication No. 2006-135748 contains a plurality of correction coefficients for residual image correction in accordance with various conditions, for example, environmental conditions such as the temperature of an X-ray imaging device and X-ray exposure conditions such as X-ray dose.
As described above, the attenuation of the residual images differs according to the time elapsed from the previous X-ray exposure (time lapse between the previous X-ray exposure and the present X-ray exposure), environmental conditions of the X-ray imaging device and X-ray exposure conditions as parameters. The Japanese Patent Laid-Open Publication No. 2006-135748 discloses to contain a plurality of correction coefficients corresponding to the above-described parameters. However, no specific description is provided. There are enormous numbers of correction coefficients corresponding to all possible combinations of the parameters, and it is just impractical to contain all the correction coefficients in the X-ray imaging device.
First, cost for hardware resources increase due to an enormous memory capacities to store all the correction coefficients. Second, to obtain data of the correction coefficients corresponding to all the combinations of the parameters, the correction coefficients need to be measured or calculated while conditions corresponding to the parameter values are changed. Such measurement or calculation requires massive cost and time.
Even a single parameter, for example, the temperature or the storage time of the X-ray imaging device contains an infinite number of values that change continuously, increasing the number of correction coefficients accordingly. Thus, it is impractical to contain all the correction coefficients for just one parameter as is the case with multiple parameters.